iPhone/iPad/iPod app to decode GMDSS transmissions
UHF SATCOM refers to satellite repeaters that operate between 240 MHz and 270 MHz. To receive SATCOM, you need a receiver that can tune the frequency range in narrow FM (most modern scanners can do this). You also need an outside antenna, and possibly a LNA preamp.
The satellites in question are operated by the US military. They are essentially repeaters in geostationary orbit. Because they are open (no access control) they are often used by third parties, most often by people in Brazil. It is very common to hear Portuguese transmissions. One listener, who spent several years living in Brazil, described it as
Portuguese slang spoken by people who never paid attention in school
Back in 2009, 39 Brazilian pirates were busted, but the activity continues.
255.550 MHz is very heavily used by the Brazilian pirates. As I am typing this, I am also hearing pirates on 253.500, 253.750, and 262.190 MHz.
There is an excellent breakdown of all of the 250 MHz SATCOM Transponders By Satellite
While you can start with a basic outdoor scanner antenna, such as a discone antenna or other scanner antenna, many serious listeners eventually build a directional antenna, such as a helical. I will have construction information about one that I built in a future article.
Next, since the signal levels are often very weak, the use of a LNA preamp is highly recommended. I built one of the Down East Microwave Inc. GaAs pHEMT pre amp kits, and find that it really helps a lot.